Modern conventional illumination devices for microscopes, in particular for operation microscopes, commonly use halogen lamps or gas discharge lamps, and in particular xenon lamps, as a light source to illuminate the microscope's object field.
Although halogen lamps are inexpensive, they have a limited lifetime (about 50 hours), and their luminosity decreases as their operating time increases because the bulb's filament material evaporates and deposits onto the bulb. Furthermore, the intensity maximum of a halogen lamp's spectrum may shift relative to daylight according to the lamp's color temperature, possibly leading to false color impressions.
Xenon lamps are therefore used in many applications in which very bright illumination is desirable, for example operation microscopes. Xenon lamps are gas discharge lamps in which a filler gas mixture containing xenon is made to luminesce in a bulb by a gas discharge. They are distinguished by a high luminous power with a large component in the visible spectral range. In the xenon lamps used to date, the gas discharge is induced by applying a high voltage to two electrodes arranged inside the bulb. Here again, however, the luminous power decreases over the operating time because the electrode material evaporates and deposits on the bulb, the lamp failing when the electrode material fully evaporates. Based on experience, conventional xenon lamps only have a lifetime of about 500 operating hours. In operation microscopes, it is therefore possible that a xenon lamp may fail unexpectedly during a long operation.
Various proposals have already been made in order to prevent the illumination device from failing unexpectedly. For example, operating counters used in illumination devices in which a xenon lamp is the light source count the lamp's hours of operation so that operating staff may change the lamp as a precautionary measure after a certain number of operating hours have elapsed. Furthermore, DE A 10 2005 060 469 discloses a lamp changer system that switches over from a primary lamp to an auxiliary lamp during operation. Suitable sensors monitor the lamp's power and automatically initiate a lamp change when appropriate.
These measures mitigate, but do not solve, the problem of limited lifetime. Furthermore, operating hour counters provide only an approximate indication of the performance capacity, so that a lamp may fail prematurely even where an operating hour counter is provided or, conversely, may have a usable remaining life despite the counter's warning. Moreover, the replacement of a xenon lamp or another discharge lamp, and/or the provision of an auxiliary light source together with a changer mechanism, are elaborate and expensive.